We recently have shown that cloned mouse cytolytic/suppressor T cells and cloned cells with NK activity (NK cells) have ultrastructural (EM) features similar to those of immature basophils, notably prominent osmiophilic cytoplasmic granules. Cloned NK cells and suppressor T cells also resemble basophils by cytochemistry and sulfated glycosaminoglycan content. In addition, like basophils and cloned mast cells, cloned NK cells express plasma membrane receptors (FcEpsilonR) that bind IgE with high affinity. By contrast, cloned inducer T cells lack cytoplasmic granules and FcEpsilonR and resemble conventional lymphocytes by cytochemistry and EM. Taken together, these findings suggest that cytolytic/suppressor T cells, at least some NK cells, basophils, and mast cells may define a spectrum of leukocyte subsets that can express certain similarities in morphology, biochemistry, and function. We now propose: to analyze the function, biochemistry, and morphology of additional cloned or freshly isolated mast cells, NK cells and cytolytic/suppressor T cells; to attempt to modulate the function of cloned leukocytes by regulating their maturation in vitro; to analyze the role of cytoplasmic granules and secretory processes in NK cell and cytolytic suppressor T cell function in vivo and in vitro; and to test whether cloned NK cells that express FcEpsilonR can mediate IgE-dependent ADCC against tumor cells or parasites.